Embedding miniature sensors in products, systems, storage and shipping containers, and other items allows the monitoring of those items to determine health, maintenance needs, lifetime, and other item characteristics. Information from miniature chemical sensors can tell a user whether or not the item has been exposed to toxic or corrosive chemical levels that can cause damage, or has leaks of chemicals within the system.
In addition, there are increasing threats from chemical/bio agents and toxic industrial chemicals in both traditional military activities and in civilian sectors involving general public populations. This has resulted in a need for the widespread availability of instrumentation for the rapid detection of a growing number of chemical/bio agents. Military needs include historical agents such as organophosphorus and explosive chemicals, chlorine diphosgene choking gas and mustard gas blister agents, blood agents such as arsine, cyanogens or hydrogen chloride, nerve agents such as soman, tabun, or sarin, mycotoxin agents such as aflatoxin, botulinus, ricin, saxitoxin, trichothecene, or toxin producing bacteria such as anthrax. Peacetime and civilian chemical/bio agent detection interests include street drugs, environmental pollutants, disease outbreaks and leaking chemicals associated with a wide variety of containment vessels such as mobile and stationary storage tanks. Detection devices suitable for chemical/biological detection have enormous potential for application to civilian and government sponsored research and development activities, exploration, and commercial industry.
The growing number of potential chemical/bio agents and toxic industrial chemical locations, and the increasing rate of sampling engagements, have led to requirements for real-time detection, i.e. sampling times on the order of 100 ms or less. In addition, real-time detection of chemical and biological agents is becoming increasingly critical in a number of applications where the accessibility of detection instrumentation, or the ability to deliver the detection instrumentation on small platforms, is limited by power, volume and weight constraints. Furthermore, in the unmanned platforms, hand-held detectors, and portable systems currently envisioned in future military operational scenarios, the power and size requirements placed on detection systems are becoming harder to fulfill. These challenges have led to the need for novel miniature chemical/bio analysis systems, including miniature optical spectrometers. New technologies in optical sources, micro-optical integration, micro-electromechanical (“MEMS”), MOEMS and optical detectors have allowed these new miniature optical sensors, as well as enabled the incorporation of more sophisticated optical techniques into ever smaller packages.
In particular, the precise placement, alignment, and control of miniature optical components on MOEMS structures allows the extension of Fourier transform spectroscopic techniques that are dominant in infrared (“IR”) spectroscopic instrumentation designs into the visible/UV spectrum with the capability of real-time detection of chemical and biological agents from small-unmanned delivery and reconnaissance platforms.
Several optical techniques have demonstrated an ability to uniquely identify chemical/bio agents, including fluorescence, emission, and absorption optical spectroscopy. Historically, absorbing spectroscopic techniques have dominated spectroscopic instrumentation from the ultraviolet (“UV”) to the IR regions because both emission and fluorescence spectroscopy require the use of intense light sources at specific wavelengths to excite the electrons in chemical molecules to higher energy states where they decay with characteristic frequencies. This typically requires the use of large inefficient frequency doubled or tunable dye lasers to reach UV wavelengths, and can often be plagued by high intensity phenomena such as Raleigh scattering.
Absorption spectroscopy, however, can be used to measure chemical molecules with electron energy state transitions that occur in UV (200 nm to 400 nm) and visible wavelengths (400 nm to 800 nm), and is typically accomplished using optically dispersing elements such as prisms, or more commonly, diffraction gratings. However, diffraction spectrometers require implement slow mechanical scanning of an optical beam across a detector, or the use of a large linear electronically scanned detector array. A diffraction grating based design is typical of the state of the art spectrometer size reductions typified by the hand-sized or personal computer add-in card sized spectrometers offered commercially by various vendors. Absorption spectroscopy at longer near-IR (“NIR”) to IR wavelengths (1,000 nm to beyond 30,000 nm or 30 um) is typically used to detect optical absorption associated with lower energy level molecular vibration excitation levels. This field of spectroscopy has been dominated by Fourier transform spectroscopy for many years because of several advantages over diffraction based optical designs. Fourier transform spectroscopy has greater optical efficiency, increased speed since the complete optical spectrum is measured simultaneously with an interferometric technique, increased sensitivity by allowing multiple scans, and reduced maintenance because it requires no external calibration and is mechanically simple with only linearly moving parts.
Historically, however, even though Fourier transform techniques are often preferred, they have been difficult to apply to UV and visible miniature spectrometer designs because of the strict demands on the precision of optical component placement, component movement, and system control mandated by operating at the shorter wavelengths. Although there have been large-scale designs (designated UVFT) proposed by university researchers, none are currently commercially available or known to have been actually fabricated and tested. However, emerging MOEMS technologies using lithography and etching techniques typical in semiconductor chip manufacturing enable the assembly of micro-optical components on physical features with tolerances of only a few hundred nanometers (10−7 meters). In addition, electromechanical actuators can be fabricated in MOEMS structures that are capable of controlling and measuring motion to the same degree of precision. These new technologies can enable the development of UVFT spectroscopy and its application to chem/bio detection in unmanned systems, hand-held detectors, and portable analytical instruments.